As the data areal density in hard disk drive (HDD) writing increases, write heads and media bits are both required to be made in smaller sizes. However, as the write head size shrinks, its writability degrades. To improve writability, new technology is being developed that assists writing to a media bit. One approach that is currently being investigated is microwave assisted magnetic recording (MAMR), which is described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. Magn., vol. 44, pp. 125-131 (2008). Although MAMR has been in development for a number of years, it is not shown enough promise to be introduced into any products yet. In particular, a difficult challenge is to find a spin torque oscillator (STO) film that is thin enough to fit into the small write gap required for state of the art products while providing a high magnetic moment in the oscillation layer to generate a sufficient radio-frequency field for the assist effect.
Spin transfer (spin torque) devices are based on a spin-transfer effect that arises from the spin dependent electron transport properties of ferromagnetic-non-magnetic spacer-ferromagnetic multilayers. When a spin-polarized current passes through a magnetic multilayer in a CPP (current perpendicular to plane) configuration, the magnetic moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high.
In a PMR writer, the main pole generates a large local magnetic field to change the magnetization direction of the medium in proximity to the writer. By switching the direction of the field using a switching current that drives the writer, one can write a plurality of media bits on a magnetic recording medium. Magnetic flux in the main pole proceeds through the ABS and into a medium bit layer and soft underlayer (SUL). In some common designs, the flux returns to the write head through a trailing side loop comprised of a trailing shield structure, and through a leading side loop that includes a leading shield and back gap connection. There is also a gap field that exits the main pole through the write gap, side gaps, and leading gap, and is not directly responsible for writing. Therefore, an improved gap design surrounding the main pole is desirable where the gap field is minimized during a write process and thereby forces a greater portion of magnetic flux to exit the main pole tip at the ABS for enhanced writability. This enhanced magnetic field does not necessarily need to coexist with a microwave assisted recording effect.